1. Field of the Invention
The present invention relates to a switch control circuit and a switch control method of an ATM switchboard, particularly to a switch control circuit and a switch control method of an ATM switchboard for switching an ATM (Asynchronous Transfer Mode) cell between input and output ports.
2. Description of the Related Art
Conventionally, the above type of ATM switchboard, as shown in FIG. 16, comprises an input port buffer 5, output port buffers 2-1 to 2-n (output port buffers 2-4 to 2-n are not illustrated), a buffer occupancy value measuring section 3, and a back-pressure outputting section 6. Moreover, the input port buffer 5 comprises a separator 50 for each port, output port corresponding logical queues 51-1 to 51-n, a cell transmission control section 52, and a back-pressure receiving section 53. For the above ATM switchboard, a case is assumed in which one traffic class is used in order to simplify the description.
FIG. 17 is a flow chart showing the back-pressure control by the back-pressure outputting section 6 in FIG. 16. The cell switching operation of a conventional ATM switch board is described below by referring to FIGS. 16 and 17.
The destination output port of an ATM cell incoming from an input port 100 is identified by the separator 50 for each port in the input port buffer 5 and then stored in proper output port corresponding logical queues 51-1 to 51-n in accordance with the identified result. The cell transmission control section 52 controls transmission of the ATM cell in accordance with the rotation preferential control of cyclically carrying about a cell transmission right between the output port corresponding logical queues 51-1 and 51-n.
ATM cells fetched from the output port corresponding logical queues 51-1 to 51-n selected in accordance with the processing by the cell transmission control section 52 are stored in the output port buffers 2-1 to 2-n corresponding to the destination output ports via an input port signal line 101 and a time-division multiplexing bus 102.
In the output port buffers 2-1 to 2-n, cells are successively transmitted to output ports 103-1 to 103-n output ports 103-4 to 103-n are not illustrated) starting with the first cell. The buffer occupancy value measuring section 3 observes the queue length of each of the output port buffers 2-1 to 2-n.
When the back-pressure outputting section 6 refers to the queue length information of each of the output port buffers 2-1 to 2-n of the buffer occupancy value measuring section 3 (step S31 in FIG. 17) and detects that the output port buffers 2-1 to 2-n brought under a congested state because queue lengths exceeding a preset threshold are present (step S32 in FIG. 17), it transmits a transmission stop (STOP) signal 113 specifying a congested output port and a transmission restart (GO) signal 111 specifying output ports other than the congested output port to all input port buffers 5 (steps S33 and S35 in FIG. 17).
Moreover, unless the back-pressure outputting section 6 detects the output port buffers 2-1 to 2-n brought under a congested state because queue lengths exceed a threshold are present (step S32 in FIG. 17), it transmits the transmission restart (GO) signal 111 specifying all output ports to all input port buffers 5 (steps S34 and S35 in FIG. 17).
The back-pressure receiving section 53 receiving the transmission stop (STOP) signal 113 specifies an output port transmitting a back-pressure signal out of received back-pressure signal information and notifies the cell transmission control section 52 so as to inhibit transmission of cells from the output port corresponding logical queues 51-1 to 51-n corresponding to the output port.
When the congestion of the output port buffers 2-1 to 2-n is cancelled, the back-pressure receiving section 53 transmits the transmission restart (GO) signal 111 specifying the output port and restarts the transfer of cells to output ports in which transmission has been stopped.
As described above, to control the traffic between an input port and an output port of a conventional ATM switchboard, only the simple back-pressure control is present in which a back-pressure signal for commanding stop/restart of the output of a cell to a specified output port is issued to all input port buffers 5 in order to prevent a cell loss in the output port buffer of the output port when the specific output port is congested.
Moreover, for the input port buffer 5, a technique of strictly controlling transmission of cells by transmitting a cell from each logical queue and then computing the next cell transmission time is considered instead of the rotation preferential control. However, either of the following two cases are conventionally used: a case of comparing the above transmission time with the present time and transmitting cells when the transmission time is earlier than the present time and a case of transmitting cells at random in accordance with a transmission time sequence without comparing the transmission time with the present time.
Problems to be Solved by the Invention
In the case of the conventional back-pressure control described above, if the congestion due to simultaneous arrival of ATM cells at the same output port from a plurality of input ports occurs, a transmission stop signal for preventing a cell loss in an output port buffer is output. Then, when the congestion is released and a transmission restart signal is output, the transmission stop signal is output again because the congestion due to the simultaneous arrival recurs though input port buffers simultaneously restart cell transmission.
As described above, in the case of the conventional back-pressure control, each input port buffer synchronously repeats transmission restart and transmission stop and therefore, the throughput from each input port to the same output port is uniform. Particularly, when the original traffic value bound for the same output port fluctuates between input ports, VCs (Virtual Channels) using the same output port may not be able to compensate the throughput at all though they reserve bands.
In the case of the cell transmission control performed by an input port buffer, the next transmission time is computed whenever transmitting a cell. However, when performing the control of continuously comparing a transmission time with the present time and transmitting a cell when the transmission time is earlier than the present time, a state in which no cell can be transmitted from an input port buffer occurs though no output port buffer is congested. Therefore, a problem of inefficient use of an output port buffer occurs.
Moreover, when performing the control of not continuously comparing a transmission time with the present time, the problem of inefficient use of an output port buffer is solved. However, congestion frequently occurs because cells are sent from an input port buffer at random and the problem due to the conventional back-pressure control starts having an influence.